Phase modulation in communications systems may generally be accomplished in one of three ways. The first technique is direct modulation of the transmitter oscillator, typically using a voltage-controlled oscillator (VCO) in a phase-locked loop (PLL) as illustrated in FIG. 1. The desired baseband signal 150 containing signalling information is summed in a summer 130 with an error voltage 180 from a phase detector/comparator 120. This composite signal is then conditioned by low pass filter 140 before being applied to VCO 160. In theory, phase changes in the baseband signal can be preserved through summation and filtering to generate a voltage that regulates the VCO 160 to phase modulate as well as phase lock the RF output signal to the reference oscillator frequency.
In practice, however, this PLL circuit 100 exhibits poor accuracy since it is difficult to determine and generate the specific voltage required to regulate the VCO 160 to obtain the desired phase modulation. Errors introduced by both the summer 130 and the VCO 160 make it very difficult to achieve accurate phase modulation. This accuracy problem is exacerbated by component tolerances, aging, temperature, DC offset in driver amplitudes, and the frequency response of each of the component devices. Thus, the circuit of FIG. 1 fails to provide accurate phase modulation.
Accurate phase modulation can be achieved using a quadrature modulator as illustrated in FIG. 2. Quadrature modulator 200 accepts two modulating inputs: the I, or in-phase component 210, and the Q, or quadrature component 230. The quadrature relationship between these components is achieved by splitting a common reference signal generated by local oscillator 220 with a quadrature hybrid 280 and feeding the resulting orthogonal signals 240, 250 as respective local oscillation signals into in-phase 260 and quadrature 270 mixers. The outputs of the mixers 260, 270 are then summed at summer 290 to form the phase modulated output signal 295.
A major drawback of the quadrature modulator 200 is the generation of phase and amplitude imbalances between the two paths. These imbalances result in leakage of the local oscillator 220 to the modulated output 295 which generates intermodulation products. The imbalances and intermodulation responses manifest themselves as undesired amplitude (AM) and phase modulations (PM) in the phase modulated signal output 295.
Another approach to the accuracy problem is direct digital synthesis (DDS). However, practical direct digital synthesizers generate relatively high levels of spurious outputs, consume more power relative to other approaches, and are generally unsuitable for many narrowband applications.
The phase modulation problems described above are particularly troublesome in narrowband digital communication systems that seek to minimize the bandwidth occupied by transmitted signals. For example, narrowband communication systems often employ saturated (or non-linear) RF power amplifiers because of their relatively high efficiency. Unfortunately, non-linear amplification of amplitude varying signals generates spectral components outside the carrier fundamental frequency. This spectral spreading is particularly undesirable in narrowband communications.
To maintain high efficiency transmission in narrowband communications environments, it is important therefore to avoid amplitude modulation of the carrier signal. Consequently, the carrier is usually modulated with the input information signal using phase or frequency modulation techniques to preserve the constant amplitude envelope of the carrier. While in theory, phase modulation does not affect the amplitude of the input signal, practical phase modulation techniques, e.g. quadrature modulation, suffer residual phase and amplitude modulation imbalances between their in-phase and quadrature paths that lead to the above-described generation of undesired spurious AM and PM components in the output of the modulated signal as described above.
Therefore, when quadrature modulators are used with high efficiency nonlinear amplifiers, it is likely that some amplitude component will be added to the carrier. As a result, undesirable spreading of the transmission spectrum occurs defeating the intent of narrowband signaling. Although the PLL modulator of FIG. 1 provides constant envelope modulation, its poor accuracy as a phase modulator makes it an undesirable alternative.
The present invention accomplishes both of these seemingly incompatible goals of efficiency and narrowband transmission. The present invention provides a quadrature modulated, phase-locked loop for use in a narrowband communication system that does not generate AM or undesirable PM components. A quadrature modulator is either placed within (or fed into) the feedback path of a phase locked loop. The phase modulated feedback signal is compared to a reference frequency signal in a phase comparator such as a digital synthesizer. Since the synthesizer only compares the relative phases of-the input signals, the spurious AM components of the quadrature modulator output are effectively removed. Thus, accurate phase modulation is achieved while still maintaining a constant envelope carrier signal and a subsequent amplification of the modulated carrier by a nonlinear power amplifier does not result in transmission of signals outside the desired bandwidth. The present invention therefore is particularly advantageous for narrowband communications.
A primary phase-locked loop includes an oscillator for generating an output signal in response to a regulating signal. A modulator combines a feedback signal from the output signal with an information signal to produce a modulated signal. A phase comparator compares the phase of the modulated signal with the phase of a reference frequency signal to produce the regulating signal. A first filter is connected between the modulator and the phase comparator for conditioning the modulated signal. A second filter is connected between the phase comparator and the oscillator for conditioning the regulating signal. A sampling means samples the output signal to generate the feedback signal. The modulator may be a quadrature modulator, the sampling means a coupler, the oscillator a voltage or current controlled oscillator, and the phase comparator a digital synthesizer.
A frequency scaler for prescaling the frequency of the feedback signal may be connected in the feedback loop before or after modulation of the feedback signal. The frequency scaler includes a variable local oscillator and a mixer for mixing the local oscillator output signal and the feedback (or modulated) signal. The variable local oscillator may employ a secondary phase-locked loop and includes means for changing the frequency of the variable local oscillator which then tunes the primary phase-lock loop to different frequencies. A low pass filter is provided to minimize phase noise in the primary phase locked loop caused by changes in the local oscillator frequency.
In other embodiments of the present invention, the phase-locked loop includes a reference oscillator for generating an oscillator output signal in response to a regulated signal. A modulator combines the first reference signal and an information signal to produce a modulated signal. A mixer mixes the feedback signal and the modulated signal, and a phase comparator compares the phase of an output signal of the mixer with the phase of a second reference signal to produce the regulated signal. In yet another embodiment, the modulator combines a referenced signal and an information signal to produce a modulated signal, and the phase comparator compares the phase of a feedback signal with the phase of the modulated signal to produce the regulated signal.
The present invention also includes a phase modulation method for maintaining a constant envelope oscillator output signal generated by an oscillator in a phase-locked loop (PLL). The oscillator output signal is sampled to generate a PLL feedback signal which is then modulated with an information signal. The phase of the modulated signal is compared with the phase of a reference signal to determine the difference, and the difference is used to regulate the oscillator. The result of the method of the present invention is that the oscillator output signal is substantially free of spurious amplitude modulation.
In another embodiment, a constant envelope reference oscillation signal is phase modulated with an information signal. Any spurious amplitude modulations generated in the phase modulation are translated into spurious phase modulation components. Thereafter, the phase of the phase modulated signal is compared with the phase of a reference signal to generate a control signal for controlling the phase locked loop, wherein the spurious phase modulation components are removed in the comparing step.